Immunoglobulin A (IgA) is not only an important antibody for mucosal immunity but also constitutes the second major antibody class following immunoglobulin G (IgG) in blood and works to defend against bacterial or viral infections. IgA includes secretory IgA (sIgA) having a dimeric structure and IgA having a monomeric structure (mIgA). sIgA has a dimeric structure in which the monomeric units are linked by a joining chain (J-chain) through disulfide bonds, and is secreted into mucus, while mIgA is mostly found in blood. Also, IgA has two subtypes: IgA1 and IgA2, which differ mainly in the length of the hinge region. IgA2 is deficient in a Pro-rich region of 13 residues. The functions of IgA directed to pharmaceuticals have been focused on the development of mucosal vaccines because of its importance for immunity against infections (Non-Patent Literatures 1 and 2). IgA in blood has been reported to have ADCC against cancer cells particularly mediated by neutrophils (Non-Patent Literatures 3 and 4). IgA can thus be expected as a cancer-targeting antibody drug, as in IgG, which is an antibody drug format whose clinical application is being expanded as a therapeutic drug for cancer or autoimmune disease (Non-Patent Literature 5).
However, there are some impediments to the pharmaceutical development of IgA, including the absence of a purification method that can work on an industrial or pharmaceutical scale as in protein A/G affinity columns for IgG production. Some methods have previously been reported as methods for purifying IgA (Non-Patent Literature 6). The reported methods for purifying IgA utilize, for example, Jackalin, a lectin recognizing an IgA1-specific sugar chain (Non-Patent Literature 7) or a protein A-mimetic synthetic ligand TG19318 (Non-Patent Literature 8). These methods are limited in use due to problems associated with binding ability or specificity. IgA-binding proteins have been found from members of the family of NI proteins (Non-Patent Literature 9), surface proteins derived from Streptococcus bacteria (Non-Patent Literatures 10 and 11 and Patent Literature 1). These IgA-binding proteins, however, have, for example, unfavorable interactions with other proteins in serum, such as IgG (Non-Patent Literature 12) and have failed to be used as IgA-specific affinity ligands. Meanwhile, Sandin et al. reported that they isolated a domain peptide (Streptococcal IgA-binding peptide, Sap) consisting of 48 residues in the Streptococcal Sir22 (M22) protein and formed a disulfide-bonded dimer thereof via Cys to obtain an affinity ligand for IgA purification having relatively high affinity (Kd: 20 nM), albeit lower than the affinity of the original Sir22 protein (Kd: 3 to 4 nM; Non-Patent Literature 13 and Patent Literature 2). In actuality, this ligand was capable of binding to IgA Fc and was also applicable to the purification of both sIgA and mIgA and the detection of antigen-specific IgA1 and IgA2 monoclonal antibodies.
Also, for IgG, IgG-binding peptides have been developed by the present inventors (Patent Literature 3), as in the IgA-binding proteins.